A wireline cable winch drum traditionally consists of a cylindrical core and two spaced apart flanges disposed at opposite ends of the core and extending radially outwardly in a direction perpendicular to the axis of the core. The winch drum functions to store wireline cable on the core in the space between the flanges, and also to convert a rotational motion of the drum into a translational motion of the wireline cable by rotating the drum about its central axis.
FIG. 1 is a partial view of one quadrant a typical winch drum 10, showing one of the flanges 12, the core 14, and a wireline cable 16 wrapped around the core 14. As shown, when the cable 16 is wound around or spooled onto the drum 10, it forms multiple layers stacked upon each other. For example, when the cable 16 is spooled onto the drum 10, a first layer 1 is formed along the outside diameter of the core 14 until the entire width of the core 14 is occupied, a second layer 2 is then formed along the outer surface of the first layer 1, and each successive layer (3-6) is formed along the outer surface of the previously formed layer.
Each layer of wound cable 16 places two primary forces on the winch drum 10: forces directed radially inwardly on the core 14 (for example forces FR1-FR3 in the depiction of FIG. 1), and forces directed axially outwardly on the flanges 12 (for example forces FA1-FA5 in the depiction of FIG. 1), with each layer increasing the cumulative forces exerted on the drum core 14 and the flanges 12.
The forces FA1-FA5 on the drum flanges 12 are primarily in the axial direction due to the fact that the flanges 12 are perpendicular to the longitudinal axis of the core 14. Although, the radial forces FR1-FR3 on the core 14 are damaging, it is typically these axial forces FA1-FA5 on the flanges 12 which cause the drum 10 to fail, and in particular it is the junction 18 between the core 14 and each flange 12 where the drum 10 is most likely to fail. This is due primarily to the large bending moment M that is created at the junction 18 by the axial forces FA1-FA5.
In addition, each successive layer increases the cumulative moment M at the junction 18 and each successive layer produces a moment at the junction 18 that is generally larger than the moment created by the previous layer due to the increased distance (or moment arm) of each successive layer from the junction 18. For example, the bending moment at the junction 18 from the first, third and fifth layers of cable 16 is equal to FA1*d1, FA3*d3, and FA5*d5, respectively. As such, the moment at the junction 18 created by each layer is directly proportional to the distance of that layer from the junction 18. This is of particular concern for wireline cable winch drums 10, since the cable 16 wound thereon can be 30,000 feet long or more. Thus, a large number of layers are required to spool all 30,000 feet of cable 16 onto the drum 10, sometimes as many as thirty layers or more. As such, the distance from the outer most layer of cable 16 to the junction 18 can be relatively large. Resulting in a correspondingly large bending moment on the junction 18, which can ultimately lead to the failure of the drum 10.
Accordingly, a need exists for a cable winch drum better suited for absorbing the forces exerted thereon for wireline cable applications and/or a system for monitoring physical properties of the winch drum.